1. Field of the Invention
The present invention relates to a digital transmission system and a digital transmission method used therein and in particular, to a transmission system having a regular line and a spare line and using a capacity variable transmission system.
2. Description of the Related Art
Conventionally, a microwave digital transmission system has a spare line to prevent a line failure caused by the fluctuation of a propagation path. When a failure occurs in a regular line, a transmission is switched to the spare line so as to continue transmitting.
Besides, the digital transmission system used for a regular line and a spare line transmits data at a fixed, guaranteed transmission capacity. The guaranteed transmission capacity is so designed that the quality is guaranteed even in the extremely bad conditions which may cumulatively occur at a rate of about a few percent a year in the whole system including propagation paths and interference.
Accordingly, during the time excluding the time of bad conditions, that is, almost every usual time, the system is not fully utilized even though it has a potential to transmit a capacity exceeding the guaranteed transmission capacity.
FIG. 5 shows an example of the structure of the conventional microwave digital transmission system. In FIG. 5, a station A-c 5 is composed of transmission switching devices 54 and 55, transmitters 51 to 53, and a signal generator 56. A Station B-c 6 is composed of reception switching devices 64 and 65, a judging circuit 66, receivers 61 to 63, and a termination device 67. This microwave digital transmission system has two regular lines and one spare line.
Input signals on a first regular line Reg1 are input into the transmitter 52 via the transmission switching device 54. The input signals are digitally modulated in the transmitter 52. The input signals modulated in the transmitter 52 propagate through a propagation path 102 and are received at the receiver 62. The signals received at the receiver 62 are digitally demodulated in the receiver 62 and the original signals are output. The original signals demodulated in the receiver 62 are output via the reception switching device 64. The same process of the above is performed for signals input onto a second regular line Reg2, using the transmission switching device 54, a propagation path 103, the receiver 63, and the reception switching device 65.
The capacity transmitted via the regular lines and the spare line is the fixed, guaranteed transmission capacity. When the signal quality of the guaranteed transmission capacity transmitted via the regular line deteriorates, or a failure occurs in a regular line, the signals on the regular line are switched onto the spare line so that the quality is ensured.
The receivers 61 to 63 output signals showing the quality deterioration to the judging circuit 66 when the quality deterioration of the monitored transmitting signals exceeds the predetermined threshold value. The judging circuit 66, when received the signals from the receivers 61 to 63, outputs switching signals (including switching signals d3 and d4) for controlling the transmission switching devices 54 and 55 and the reception switching devices 64 and 65.
For example, when the spare line (especially the propagation path 101) is in the normal state (the signal is not output from the receiver 61) while a failure occurs in the first regular line Reg1, that is, the signal is output from the receiver 62, the judging circuit 66 outputs a switching signal d3 in addition to the switching signals. Once the transmission switching device 54 and the reception switching device 64 receive the switching signals from the judging circuit 66 and perform line switching, the input signals on the first regular line Reg1 are switched onto the spare line (101). As a result, the quality of the signals, which are to be transmitted via the first regular line Reg1, is prevented from deteriorating.
As described above, the conventional transmission system is a system which transmits a fixed, guaranteed transmission capacity via a regular line. A spare line in the system is used to transmit input signals on the regular line when the transmission quality of the guaranteed transmission capacity of the regular line deteriorates.
Accordingly, the regular line is not capable of transmitting a capacity exceeding the guaranteed transmission capacity even when the propagation conditions of the transmission system are in the good state. Further, since the spare line is used only when a failure occurs, the frequency of use is low. Therefore, the system is not efficient as a transmission system for transmitting a large amount of information.